Disposable devices have been provided for conducting immunoassay tests for pregnancy and/or infectious diseases. Examples are described in U.S. Pat. No. 3,888,629. In such devices, there is a filter used to separate an upper reaction compartment from a lower compartment used to collect liquid that passes through the filter. Reagents comprising antigens or antibodies are often stored in the upper compartment, to react with an antibody or antigen, respectively, at the filter to produce a complex which is retained on the filter. The solvent passes through, carrying with it free, unreacted antigens and antibodies. Most preferably, the filter is sized to hold back just the complex that forms in the immunoassay of choice.
A problem has arisen in preparing such a device. Ideally, the reagents are applied to the upper chamber by coating them onto the top surface of the filter. However, the filter is designed to allow liquid to pass through containing such reagents in their uncomplexed form. Attempts to quickly dry the reagents have met with damage to the reagents if the drying temperature is too high. Thus, it has not been possible to apply the reagents as an overcoat, because the reagent solution passes through the filter before the solvent can be dried to leave the reagents on the top of the filter (or slightly penetrated into the top surface.) What has been desired is a filter that permits the reagents to be dried in place, and also permits eventual passing of liquid through the filter.
U.S. Pat. No. 4,587,102 teaches a test element comprising stacked layers of porous fabric, wherein the void volume is preferably 50 to 90%. A time-delay layer is added to the detection layer, col. 12. It comprises one or two water soluble polymers, which can be polyvinyl alcohol, polyvinyl pyrrolidone, or carboxymethyl cellulose. The amount is said to be from 2 to 50 microns in thickness, col. 13. These amounts in turn provide a dry coverage that is between about 2.2 g/m.sup.2 to about 55 g/m.sup.2. No suggestion is made that a particular induction time be achieved or that drying temperatures might create a problem.